Concerning the heat-resistant polymer, fluorine-containing materials and aromatic/heterocyclic polymers such as polyimidebendazol, aromatic polyamide and polyimide have been studied since a silicone was synthesized by Rocho, et al. Further, in recent years, as astronautical and aeronautical fields are developing, a demand for heat resistance is increasing higher. As a result, research has been progressed in aromatic/heterocyclic polymers, and an improvement of aromatic polyimide is being proceeded. Besides, organic metal polymers, the main chain of which comprises metal elements such as Si, Ti and B, and O, N, etc. such like polyborosiloxane and polytitanosiloxane, are being studied.
Recently an improvement of heat resistance is being demanded in various heat-resistant and insulating material fields. For example, as one of the uses of heat-resistant and insulating materials, a field of a rotating machine such as a generator, a high voltage alternating current motor, a direct current motor for industrial use, an electric train motor, etc. are exemplified. Classified ones in these high voltage rotating machines have a tendency to be made large volume or high voltage and therefore downsizing and high heat resistance thereof are also required. In addition to that, the specific property required heat resistance in these fields is 400° C. or higher in the air. Additionally, an electric insulating property, flexibility, a mechanical property, etc. are also required together with the heat resistance.
As the other applications of heat-resistant and insulating materials, there are raised various ones including an insulating or dielectric layer of a display device such as a dielectric layer for PDP, an interlayer dielectric for a semiconductor, materials for semiconductor elements such as Pre-Metal-Dielectric Layer (PMD) material used suitably upon formation by a low temperature process of 600° C., a front panel of display equipment, a heat resisting film and a heat resisting sheet which are used for a various applications, a heat resisting coating, a plastic optical fiber, an electric wire sheathing, etc. In these applications, optical and electric properties such as transparency, a dielectric property, etc. are also required in addition to heat resistance, flexibility and mechanical property described above. Further, there are being tried applications as materials in the fields of ceramic composite materials, ceramic-metal adhesives, hybrid products with a ceramic material such as Al2O3, MgO, etc., and hybrid products by blending other polymers. However, the heat resistance temperature being made practicable is 300° C. or less concerning to an organic polymer in the heat-resistant polymers being currently used.
On the other hand, organic metal polymers such as silicone resin, polyborosiloxane resin, etc. have high heat resistance of 500° C. or higher since the heat stability of the polymers is high. However compared to organic polymers of which molecular structure is linear structure with less branches, the organic metal polymers have network structure with many branches to form gigantic three-dimensional network structure by the dehydrogenation condensation reaction between an OH group and another OH group or an oxidation reaction. Consequently the flexibility of the organic metal polymers got poor and then the use of the organic metal polymers has been limited.
Therefore in order to improve such defects of the organic metal polymers, hybridization of an organic resin and an inorganic material or the like are being tried. For example, block copolymerized silazane consisting of an inorganic polysilazane part and an organic polysilazane part is being proposed (for example, Japanese Patent Publication Laid-open No. Hei 2-175726). However such hybrid polymers have such problems as insufficient mechanical property etc. and were not still in a satisfactory level.
Considering such situation, the present inventors developed a high heat-resistant silicon-containing copolymer comprising silazane copolymer containing particular recurring units (Japanese Patent Publication Laid-open No. Hei 8-231727 and Japanese Patent Publication Laid-open No. Hei 9-188765). This silicon-containing copolymer is an organic metal polymer which has the heat resistance of 400° C. or higher and is excellent in a mechanical property and flexibility. In addition, its production is easy and the copolymer has such characteristic as the transparent polymer being formed upon curing in nitrogen. Therefore, it is a material that is useful for high heat-resistant optical use. However there is a problem that the cured polymer is discolored when cured in the air and it is difficult to obtain a film which is good in transparency. Because of this, it is necessary to cure the silicone-containing copolymer in nitrogen in order to obtain a transparent cured polymer, and occasionally it is difficult to apply for the some uses from the viewpoints of process cost and mass production.
Because of this, in order to increase the usefulness of the silicone-containing copolymer more, it is a further theme to develop a silicon-containing copolymer which can realize a comparable or higher transparency even when curing-treatment is made in the air without losing a heat resistance to/than that when curing-treatment is made in nitrogen. As a silicon-containing copolymer, it is also required to provide with the polymer which is not only excellent in heat resistance, a mechanical property, flexibility and transparency but also is low in a relative dielectric constant.
The object of the present invention is to provide a silicon-containing copolymer which solves the above described problems, it means, which has heat resistance of 400° C. or higher, is excellent in mechanical strength and flexibility, has high and colorless transparency even when cured in the air and also a low relative dielectric constant and to provide a production method thereof.
And another object of the present invention is to provide a film which is formed with the silicon-containing copolymer having the aforementioned excellent properties and is not only excellent in heat resistance, transparency, mechanical strength and flexibility but also low in a relative dielectric constant.